The chucks commonly used in high speed dental handpieces for releasably holding dental burs in place have chucking actions that can be generally categorized as falling within one of two groups. The first groups includes the wrench operated pull-to-tighten and push-to-tighten types of chucks. The second group includes the spring grip chuck. While efforts have been made to develop a fully satisfactory chuck which overcomes certain disadvantages that have come to be associated with chucks of these types, such efforts have not been entirely satisfactory.
Both wrench operated pull-to-tighten and push-to-tighten chucks tend to undergo bur "walkout" or ejection during conditions of extremely heavy cutting particularly in tenacious types of materials such as gold alloys and some silver amalgams under which the cutting dynamics are very severe. Test work has shown such axial walkout may occur even without any accompanying torsional slippage of the bur. Furthermore, such wrench operated chucks occasionally present a further problem, that of the chuck unscrewing from the rotor and possibly releasing during handpiece operation.
A variation of the push-to-tighten chuck is a double ended type with a separate nut. Advantages in elimination of bur walkout can generally be expected with this type of chuck but definite disadvantages include the nut loosening during cutting with the possible result being that the chuck, bur, or tightening nut will unscrew and either jam the handpiece or even possibly allow the loose pieces to fly through the air with injurious velocity.
The spring grip type chucks heretofore known also have the advantage of generally overcoming the problem of bur walkout or ejection but are commonly associated with other troublesome disadvantages. In the use of a spring grip chuck, the bur is normally removed by utilizing a small diameter push rod which the operator must direct with a force strong enough to overcome the spring and frictional forces. Conversely, in order to insert the bur into this type of chuck, the bur must generally be placed against a soft brass or plastic slug so it can be pushed into place without damage to the bur surface against which the slug is pushed. Major disadvantages of spring grip chucks have resided in the fact that it is often difficult and dangerous for an operator to apply sufficient force to overcome the springs for bur insertion and removal without damaging the bur, the handpiece, or both.